1. Field of the Invention
The present invention relates to a vibration detector usable notably for the reception of acoustic or seismic waves.
2. Description of the Prior Art
There are well-known vibration detectors comprising at least one sensitive element of piezoelectric type or of another type, fastened to a flexible wall of a housing. This wall is for example the central part of a plate whose peripheral part is reinforced and rigid. The housing can be defined by two analogous plates resting against one another by their thicker peripheral parts and each bearing a sensitive element. The electrodes of the two sensitive elements are interconnected electrically so as to ensure a compensation of the parasitic effects due to accelerations. When the outside static pressure increases, the two plates bend until they rest against one another. The space between them is so selected that their maximum deformation remains within their elastic deformation limits when they are pressed against one another. The detector is thus protected against accidental overpressures.
The sensitive elements can be fastened outside the housing, which gives them a sensitivity that varies little (less than 10% for a static pressure of 10 MPa for example) with the hydrostatic pressure variations. The sensitive elements are generally covered with a protective coating (such as a varnished araldite coat) so as to maintain a sufficient electric insulation between the electrodes. This layout provides very sensitive detectors at a relatively low cost, but their reliability may sometimes deteriorate as a result of the direct exposure of the sensitive elements and of their possible protective coating to the outside environment.
According to another well-known layout, the sensitive elements can be fastened to the plates on the inner faces thereof and therefore inside the housing, which ensures them a better protection against the outside environment. However, with this layout, the piezoelectric sensitive elements, which are sintered ceramic disks and therefore relatively fragile, can be easily damaged by crushing when the increase in the hydrostatic pressure causes them to be pressed against one another by bending the plates. It has been observed that their sensitivity decreases substantially with the hydrostatic pressure increase as a result of a phenomenon linked with the fastening thereof on a face that becomes convex.
Various piezoelectric detectors are described for example in the assignee's French patents 1,556,971 and 2,122,675 and in U.S. Pat. No. 4,336,639.